In the past, in the automotive field, for conservation of the environment, reduction of the weight of car bodies to improve fuel efficiency and improvement of collision safety have been sought. For this reason, various attempts have been made up to now to use high strength steel sheet to reduce thicknesses and to optimize car body structure to reduce the weight of car bodies and improve collision safety.
Large fatigue strength is also being demanded from high strength steel sheet for reducing the weight of car bodies. In particular, in suspension arms, subframes, and other chassis members, the fatigue strength of the welded zones becomes more important. To weld these chassis members, fillet arc welding is often used. Therefore, to reduce the weight of chassis members, the challenge is to raise the fatigue strength of the fillet arc welded joints.
FIG. 1A shows the cross-sectional shape of a typical lap fillet welded joint formed by superposing an upper steel material 1 and lower steel material 2 and welding only the perpendicular crossing parts at one side among the perpendicularly crossing parts formed at the two sides of the abutting parts of the upper steel material 1 and lower steel material 2. In the following explanation, the fillet weld bead will when necessary be called a “fillet bead”. In such a lap fillet welded joint, if tensile loads F1 and F2 act on the upper and lower steel sheets, rotational deformation occurs about the fillet bead 3, stress concentrates at the toe 4 or the root 5 of the fillet bead 3, a fatigue crack proceeds in a direction vertical to the load, and the welded joint breaks. For this reason, to improve the fatigue strength, it becomes important to reduce the stress concentration at the toe 4 or root 5.
As the method of improving the fatigue strength of the welded joint, in the past, efforts have been made to reduce the stress concentration at the weld toe mainly by improvement of the bead shape. For example, post-processing is performed to increase the flank angle by grinding the weld toe or by remelting the toe by TIG welding. Furthermore, PLT 1 proposes to provide a slanted part at a weldable member in advance by press-forming and perform the welding so that the weld bead is superposed on that slanted part to thereby increase the flank angle of the toe of the weld bead.
PLTs 2 and 3 disclose the art of augmenting a weld bead to ease the stress concentration at the weld toe. Specifically, PLT 2 discloses the art of extending the weld bead, while PLT 3 discloses the art of forming a stiffening bead.
PLTs 4 and 5 disclose the art of treating a weld bead and smoothing the weld toe to ease the stress concentration at the weld toe. Specifically, PLT 4 discloses the art of brazing the weld bead, while PLT 5 discloses the art of using plasma to remelt the weld toe.
PLT 6 discloses the art of press-forming a weldable member so as to form a projection in advance near the location scheduled for welding where the weld bead will be formed and thereby ease the stress concentration at the weld zone. Specifically, PLT 6 discloses superposing and welding a plurality of sheet-shaped members to form a straight weld zone and forming press beads in shapes surrounding the two end parts so as to make the tensile load and torsional load applied to the sheet-shaped members be absorbed by the press beads and prevent stress from concentrating at the two end parts of the straight weld zone.
PLT 7 discloses providing an upwardly projecting bead part at a side wall part side of a lower rail of a flat sheet-shaped bracket to run along the side wall part of the lower rail. PLT 7 discloses using the upwardly projecting bead part to increase the interval between an arc welding torch and the top surface of the bracket and improve the weldability in the arc welding.
PLT 8 discloses a weld fastening structure of a lid hemming part for automobile use. PLT 8 discloses to fasten the edges of a lid outer panel folded along the edges of a lid inner panel by arc welding to a raised part of the inner panel so as to be able to sufficiently withstand the large external force at the time of collision etc.
However, these techniques have little effect on improvement of the fatigue strength of the portions where stress structurally concentrates such as in the lap fillet welded joint shown in FIG. 1A, that is, the toe 4 and the root 5. A fundamental improvement in fatigue strength cannot be realized. In particular, in a welded member of thin steel sheets, since the sheets are thin, the member falls in rigidity. Due to the carried load, the part of the lower steel material 2 of FIG. 1A which overlaps the bottom side of the upper steel material 1 easily deforms by rotation about the fillet bead 3. For this reason, a fatigue crack sometimes forms not only from the weld toe 4, but also from the root 5. With just improvement of the shape of the weld toe, the fatigue strength or fatigue life is insufficiently improved.
Further, in a thin sheet welded structure, to prevent weld deformation due to excessive weld heat input, in many cases, not the entire length of the joint is welded: The welded structure is prepared by partial weld beads 301 such as shown in FIG. 1B(a). Each of the partial weld beads 301 is formed with a beginning 301S and an ending part 301E. The conventional technique for improvement of the bead shape is art covering a middle portion 301M stable in the weld bead shape shown in FIG. 1B(c). However, as shown in FIG. 1B(b), the beginning 301S insufficiently penetrates the matrix, has a toe of an upwardly projecting shape, and easily becomes a location for formation of a fatigue crack in the toe 4S. Further, as shown in FIG. 1B(d), the ending part 301E becomes reduced in thickness of weld metal and easily suffers from cracks from the root 5E. In this way, in an actual welded structure, cracks easily form at the beginning and the end 301S and 301E of the weld bead, so no effect of improvement of the fatigue strength of the welded structure as a whole can be expected.
That is, with thin steel sheet welded members, to improve the fatigue strength of a welded structure, in particular one including a lap fillet welded joint or T-fillet welded joint, it is considered important to raise the rigidity of the members to suppress out-of-plane deformation and thereby reduce the structural stress concentration. If possible to raise the rigidity of the members and lower the structural stress concentration, it may become possible to obtain the effect of improvement of the fatigue strength of the welded structure as a whole including the beginning and root of the weld bead.
PLT 9 proposes forming an arc weld bead for stiffening use separate from a fillet bead so that the position of the weld beginning point overlaps the fillet bead and so that the position of the weld ending point becomes the front surface of the lower steel sheet whereby this functions as a member for raising the alloying of the steel sheet and weld metal in the direction to which the bending moment is applied and suppresses bending of the lower steel sheet.
Further, in the past, as means for improving the fatigue strength of a fillet welded joint, the practice has been to weld a rib or other reinforcing member with a suitable shape and position. However, if welding a separate member, this leads to an increase in costs, so these means are not art able to be applied to the production of mass production parts such as auto parts.
To deal with such a problem, PLT 10 proposes the art of fillet arc welding one metal member and another metal member to produce a welded joint during which forming on the surface of at least one of the metal members at least one bead beginning from the fillet bead and separate from the fillet bead in the same plane as the fillet bead by a required angle to thereby improve the fatigue strength of the welded joint.
However, in the arts disclosed in PLTs 9 and 10, it is necessary to form the fillet bead, then form the stiffening weld bead in a predetermined direction with respect to the fillet bead, so there are the problems that application to a part with a complicated shape is difficult, the welding steps increase, and improvement of the productivity is limited.
Further, in general, at the beginning point of a weld bead, penetration becomes insufficient and weld defects easily occur. In the arts disclosed in PLTs 9 and 10, the weld beginning point of the stiffening bead overlaps the fillet weld bead, so at the overlapping part, penetration is liable to become insufficient and a weld defect is liable to occur. On the other hand, if sufficiently heating the weld beginning point of the stiffening bead to eliminate the problem with penetration of the stiffening bead in the matrix metal, there is the problem that the matrix metal becomes excessively heated at the weld beginning point and a drop in the strength of the matrix metal is invited.